Basic knowledge of car exhaust system modification

Basic knowledge of car exhaust system modification

1. How does the exhaust system work?

The function of the exhaust system is to discharge the exhaust gas in the cylinder into the atmosphere with as little exhaust resistance and noise as possible. The exhaust system mainly includes exhaust manifold, exhaust pipe and muffler. The exhaust gas passes from the exhaust port connected to the combustion chamber, through the exhaust manifold, the front and middle section of the exhaust pipe (including the three-way catalytic converter) and the rear section of the exhaust pipe (commonly known as the tail exhaust), and is finally discharged into the outside air.

It has been mentioned in the modification of the intake system that reducing the intake resistance and exhaust resistance, increasing the pressure in the cylinder, lowering the temperature at the end of the intake, reducing the pressure of the residual exhaust gas in the exhaust pipe, and increasing the area of ​​the intake valve can all improve the engine performance. The volumetric efficiency increases the power and torque of the engine accordingly and improves the power performance. Therefore, when modifying the exhaust system, the pressure difference between the free exhaust cutoff cylinder and the exhaust pipe should be expanded as much as possible, the exhaust inertia should be fully utilized, the exhaust resistance should be reduced, the pressure of the residual exhaust gas in the exhaust pipe should be reduced, and the residual exhaust gas should be reduced. The content ultimately makes exhaust smooth and fast, improves volumetric efficiency, and obtains better power performance. In order for the exhaust system to achieve higher exhaust efficiency, it is necessary to make full use of the back pressure (back pressure) and "suck" effect generated by the exhaust system during operation.

When the engine is working, the exhaust gas delivered by the exhaust system is not continuous, but in waves; the more cylinders or higher the speed of the automobile engine, the denser the exhaust gas waves. The low pressure generated after the high-pressure exhaust gas wave is discharged from the cylinder at high speed is the back pressure. An appropriate amount of back pressure can produce a good sucking effect in the exhaust system. The principle is: if the diameter of the exhaust pipe is moderate, the first wave of exhaust gas discharged from the cylinder will press against the inner wall of the exhaust pipe and be quickly discharged downstream after expansion. At the same time, a low-pressure area will be generated in the upstream part to suck the second wave of exhaust gas discharged from the engine. Wave exhaust. When the second wave of exhaust gas presses against the inner wall of the exhaust pipe and is discharged outward, the low pressure generated behind it in turn sucks the third wave of exhaust gas, and so on, allowing the engine to complete the exhaust action effortlessly while improving the exhaust efficiency. It is also higher than not using an exhaust system at all. There is less exhaust gas remaining in the cylinder, and the cylinder can hold more fresh air for combustion. Therefore, only by making full use of the back pressure and suction effect can the exhaust system achieve higher performance.

2. Technical points for modification of exhaust system

In the modification of the intake and exhaust systems, the modification of the exhaust system is more popular, because the exhaust system, especially its rear section, is not only clearly visible outside the car, but also can emit stunning audio and sound. It is one of the few that can. Modified parts that directly provide car owners with visual and auditory stimulation. The original exhaust system is very conservative in design due to limitations such as cost, noise pollution, and different emission standards in different regions. This will, to a certain extent, limit the performance of the engine due to low exhaust efficiency. Therefore, exhaust system modifications can often bring unexpected gains to car owners.

(1) The modification of the exhaust system can follow the method from shallow to deep.

1) General exhaust system modification only involves the tail section of the exhaust pipe. The diameter of the tail section of the exhaust pipe can be appropriately increased and the structural form of the muffler can be changed to increase the airflow per unit area, thereby making the exhaust gas emissions more efficient. Smooth. The most basic modification is to put a more beautiful stainless steel sheath on the end of the exhaust pipe. The advantage is that it is cheap, easy to install, and will not have any impact on the original car.

2) For deeper modifications, you can saw off the tail section of the original exhaust pipe and weld a special tail throat at the cut-off part, or directly replace the tail throat of original special models launched by different brands. This tail throat is made of stainless steel. Its biggest feature is that it can emit a low-frequency sound. Although the sound quality is low, it is very refreshing. However, if this modification method is adopted, the car owner must abandon the tail section of the original car exhaust pipe.

3) The deeper modification focuses on the front and middle section of the exhaust pipe and the exhaust manifold. Modification of the front and middle section of the exhaust pipe involves modification of the three-way catalytic converter and the inner wall and diameter of the exhaust pipe; modification of the exhaust manifold involves the arrangement of the exhaust line.

(1) Modification of different components of the exhaust system

1) Exhaust manifold. Exhaust manifold modification is generally the most expensive part of a modified exhaust system. Due to the strict requirements on length, curvature and blind diameter, its development and production costs are high. However, the excellent design of the exhaust manifold plays a decisive role in the performance of the entire exhaust system.

Generally, for models with four-cylinder engines, there are two types of exhaust line arrangements: "Four out of one (4-1)". The four exhaust manifolds coming out of the linkage part of the four-cylinder engine are directly integrated into one exhaust manifold. The pipeline design of the pipe; "Four out, two out, one (4-2.1)", the two exhaust manifolds are first combined into one pipe, and then an expansion chamber is added to reduce the exhaust gas pressure. The exhaust gas branched out from these two parts is combined into The final exhaust pipe. The traditional concept is: the "four out of one" design will be more suitable for the exhaust gas fluctuation frequency at high speeds, which will help the car improve the power performance at high speeds; while the "four out of two out of one" design is more conducive to low speeds. hour's torque output, so it is more suitable for daily urban driving. But now many of the latest designed "four out, two out, one" exhaust manifolds can simultaneously improve the performance of the engine at medium and high speeds, so they can better meet the all-round requirements of users. In addition, the "four out, two out, one" modification method requires dismantling the original exhaust pipe and installing a new exhaust pipe. At this time, if you choose a better exhaust pipe, you can get a "charming" exhaust Noise and higher exhaust efficiency, while also taking into account appearance and durability.

2) Three-way catalytic converter. The three-way catalytic converter is an important device for controlling harmful components of exhaust gas. It mainly purifies harmful gases such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) in engine exhaust gas. In order to reduce exhaust resistance as much as possible, many tuners forcibly remove the three-way catalytic converter. This will not only cause the car's emissions to exceed the standard, but may also cause some car ECUs with self-diagnosis functions in the car to send out wrong signals, affecting normal driving. , so it is recommended to retain the original car's three-way catalytic converter. Nowadays, many new three-way catalytic converters will not hinder the exhaust flow rate much. You can replace it with a specially modified high-performance three-way catalytic converter, but the price is higher.

3) Exhaust pipe. Simply replacing the muffler will not increase the car's power performance much, but if the entire exhaust pipe is replaced with a high-performance model, it will play a significant role in improving the power (especially after the engine is modified). The high-performance exhaust pipe has a larger diameter than the original, smoother inner wall, and smaller curvature. These designs allow the large amount of exhaust gas generated by the engine at high speeds to pass smoothly and at high speed, significantly improving exhaust efficiency. However, it should be noted that an exhaust pipe with an excessively large blind diameter will affect the back pressure effect and cause the exhaust gas to cool down too quickly. In addition to slowing down the flow rate, it also reduces the ground clearance of the exhaust pipe and increases the "bottoming" of the car. Opportunity. Generally speaking, if the engine has not been significantly modified, the exhaust pipe diameter of a naturally aspirated engine with a displacement of less than 2L should not exceed 50mm, while the exhaust pipe diameter of a 2L turbocharged engine should also be 70~80mm. .

There is no need to replace the middle exhaust pipe separately. If it needs to be replaced, it needs to be replaced together with the muffler. The mid-section exhaust pipe is originally the cheapest part of the system, but the price will increase after adding a three-way catalytic converter.

4) Muffler. The muffler is easy to assemble and retrofitting is simple. High-end mufflers are made of stainless steel (or even titanium alloy). The modified muffler is lighter, more durable, and more beautiful than the original cast iron one. Mufflers can be roughly divided into two types: the first one uses staggered baffles to create reflected waves to reduce the volume. Almost all original mufflers are of this type. Its advantages are low cost and good noise reduction effect, but its disadvantage is that the exhaust resistance is large and bulky. The second type is the sound-absorbing type that is common in high-performance models and uses sound-absorbing materials such as glass wool to muffle the sound. The advantages are less flow restriction and light weight. The disadvantage is that the noise reduction effect is low, so there is generally a loud exhaust sound, but the size of the exhaust sound is not directly related to the performance of the engine. In addition, the diameter of the exhaust port at the end of the muffler must also match the diameter of the front exhaust pipe. If it is too large, it will not have any practical effect. When purchasing a muffler, if you only require improvements in appearance, loud exhaust sound, and general performance, many cheaper domestic modified mufflers will do the job. However, if you require higher performance improvements, you can choose related well-known foreign brands. The product.

Thank you SNW for providing the information. I hope it can help you learn more about auto parts and the structure of the exhaust system. SNW specializes in manufacturing The exhaust system parts mainly includes exhaust manifolds, headers, downpipes, cat-back systems,aluminum alloy intake pipes, T-pipe, Y pipe, X pipe, H pipe and other performance parts; the shock absorption system parts mainly includes coilovers and arms. If you want to know more about product models, please contact SNW, info@snwmetal.com, sales@snwmetal.com.